1. Field of the Invention
The present invention relates generally to a solid-state imaging device, and more particularly to an insulated-gate image sensor in which various circuits including imaging elements are formed on the same substrate. For example, the solid-state imaging device is applied to an amplification-type MOS image sensor.
2. Description of the Related Art
An amplification-type MOS image sensor includes a photosensitive pixel region (imaging region) and an optical black (hereinafter abbreviated as “OB”) area which is formed around the photosensitive pixel region and is shielded from light. A photoelectric conversion unit is also provided in the OB region. In a conventional amplification-type MOS image sensor, when light with high intensity is made incident on the photosensitive pixel region, in particularly, on a peripheral part of the photosensitive pixel region, carriers (electrons), which have overflowed from a photoelectric conversion unit of a unit pixel, flow into the photoelectric conversion unit of the OB region. Thereby, a dark-time level (black-reference level) varies, and blooming occurs. Specifically, when the dark-time level varies, a horizontal-stripe image occurs on a screen which is produced by reproducing an output signal of the MOS image sensor.
In order to avoid this problem, in a charge-coupled device (CCD) image sensor, an N-type substrate is used and a vertical over-drain structure is formed, and electrons overflowing from the photoelectric conversion unit are dumped in the substrate via the over-drain. On the other hand, in the MOS image sensor, various circuits, such as imaging elements, a scanning circuit, a register circuit, a timing circuit, an A/D converter, a command circuit, a D/A converter and a digital signal processor (DSP), are formed on the same substrate. In the case of the MOS image sensor, in order to realize the merit of reducing the system size and providing an inexpensive product, it is necessary to use a substrate having a conductivity type that is suited to imaging elements and other various circuits. Thus, in the MOS image sensor, unlike the CCD image sensor, it is not possible to use the N-type substrate or to form the vertical over-drain structure. Hence, there arises a problem of a variation in dark-time level.
Jpn. Pat. Appln. KOKAI Publication No. 2004-39794 discloses that a high-concentration N-well is formed in the entire OB region, thereby to prevent a variation in black-reference level.